This invention relates to a rapid process for making transparent tints that match the color and strength of a standard tint, using the light transmittance properties of the tints in the wet to bring them within an acceptable match to the standard.
Transparent tints are widely used nowadays in formulating metallic coating compositions used in particular for exterior finishes for automobiles and trucks. The transparent tints contain pigments with extremely small particle sizes (typically less than 0.4 micron in size) so that they impart the desired color to the coating without detracting from its transparency and clarity. This permits the metallic pigments, e.g., aluminum flake or pearlescent pigments, that are used in combination with the transparent tints, to show through and provide the finish with metallic glamour and maximum flop.
In the manufacture of transparent tints, one problem is to match the color and tinting strength of the tint as it is being made to a standard tint. The standard procedure used nowadays to test and bring the tints within an acceptable match to the standard is a manual process, which involves taking an aliquot of tint, blending it with a standard white paint, spraying the blend onto panels, baking the panels, and then measuring one or more color properties of the dried coating using a spectrophotometer or calorimeter. The color measurements are then compared to sprayouts of a standard batch of that tint blended with the same standard white paint to calculate the quantities of shading colorants and clear binder required to correct the batch for color and strength.
The technique for mixing a highly absorbing transparent tint with a standard white paint is necessary because the absorption of the tint, in its neat state, is so high that small differences in the hue or chroma between two such tints cannot be seen. Blending with white magnifies these differences, so that they are seen more like they would be seen in the final paint. Similarly, little information on tinting strength, which is the ability of a tint to impart its color to a final product, can be obtained without blending with a white standard. Highly scattering tints (e.g., yellows) may be blended with a standard black to determine strength by the tint""s ability to lighten the black. Furthermore, if there are hue or chroma or strength differences between the test tint blend and the standard tint blend, then other colorants or clear binder must be added to correct for these differences. Thus, the color values such as L*, a*, b* or L*, c*, h* of the test tint must be matched within acceptable tolerances using the blends rather than neat tints in order to completely control the color and strength of the test tint.
Measuring color and strength and shading the tints by this method, however, is cumbersome and very time consuming because of sample preparation and paint drying time, which causes major delays in the tint manufacturing process. Other problems also arise in that the accuracy of the test is dependent on the color and strength stability of the standard white or black paints. Even with careful control, these standards tend to vary from batch to batch and also tend to flocculate or settle in time, leading to poor test repeatability and difficulty in accurately matching the tint being made to a standard tint. Additionally, there is inherent variability in the dry sample preparation process which also leads to inaccuracy in color determination.
The aim within the industry for some time has been to measure the color and strength properties of tints and shade in the wet state and in a way which predicts the color and strength of the tint when applied and dried. The primary benefits are mainly associated with time savings although some are associated with the increased likelihood of an automated tint manufacturing process.
Yet, wet shading techniques proposed in the past have produced inconsistent results. A number of wet reflectance methods have been proposed for adjusting the color property of fluids to bring them within an acceptable match to the standard. These methods, however, are mainly aimed at opaque paints. In the case of highly absorbent transparent materials, the prior art does not teach how to go about adjusting them unless they are first blended with a standard white or black base and sprayed out and compared in the dry, which still leads to the same inaccuracies as mentioned above.
Falcoff et al. U.S. Pat. No. 4,403,866 issued on Sep. 13, 1983 teaches automatic adjustment of color properties using wet shading techniques, but does not teach how to accomplish this in transmission mode. Auad WO 98/16822 published Apr. 23, 1998 provides an instrument for measuring the color properties of colored bases or tints using wet light transmittance but fails to teach how to shade these bases and tints. A related article by Pfeil, Tinting Strength Adjustments Using Light Transmission, Paint and Coatings Industry Journal, April 2000, discusses the advantages of transmittance measurements but does not teach how to color shade using such measurements.
Therefore, there is still a need for a process for making transparent tints that utilizes a faster, more accurate and more consistent wet shading technique by light transmittance, that does not require spraying and blending with white or black standards and the production of dry samples during the manufacturing process, and allows the tints to be shaded for color and strength in a very short period of time, with confidence that the wet readings will lead to an acceptable match with the standard in the dry.
The invention relates to a rapid process for making a transparent tint that matches the color and tinting strength of a standard tint, which involves the use of an improved wet shading technique that measures the light transmittance of the tint in the wet over the visible spectrum and quickly determines therefrom with precision the amount of components of the tint which must be added to bring the tint within an acceptable color and strength tolerance which provides a good wet and dry match to the standard. The process of this invention is particularly useful in the production of transparent tints for high performance metallic coatings for automobile and truck exteriors, but can also find use in the manufacture of other paints, printing inks, ink jet inks and other colored products using transparent tints.
The process for manufacturing transparent tints involves the following:
(a) charging the components of a transparent tint such as clear polymer binder for the tint, solvent for the tint, and colorant in the form of a pigment dispersion, transparent tint or dye solution in a mixing vessel;
(b) mixing the components to form a liquid tint;
(c) shading the tint during its manufacture by passing the liquid tint through a controlled-pathlength transmittance cell coupled to a spectrophotometer;
(d) measuring the light transmittance of the liquid tint over the visible spectrum;
(e) calculating the color values such as L*, a*, b* or other color values of the wet tint being manufactured from the light transmittance measurements;
(f) comparing the color values of the wet tint being manufactured to the color values of the standard wet tint and calculating the difference between the values of the tint being manufactured and the standard tint and calculating the quantity of components to be added to the tint to bring the tint within specified color tolerance values for the tint;
(g) adding to the tint being manufactured the quantities of components calculated in step (f);
(h) repeating steps (b)-(f) at least once in the event the tint is not within the specified color and strength tolerance until the tint being manufactured is within said tolerance.
In a preferred embodiment, the process might also comprise:
(i) adjusting the pathlength of the transmittance cell so that it is the same when measuring the light transmittance of the tint being made as it was when measuring the standard tint.
In another embodiment, the process may further comprises:
(j) measuring the light transmittance in direct transmittance mode for tints without scattering pigments, and utilizing diffuse transmittance for tints containing scattering pigments.
In another preferred embodiment, the invention utilizes a computer to perform the necessary calculations. In yet another embodiment, the invention provides an automated computer controlled batch or continuous process for the manufacture of transparent tints that utilizes on-line testing of the tints.